The electrooxidation of CO on Ru (0001) and RuO2 (100) electrode surfaces were characterized by. cyclic voltammetry, AES and RHEED. The CO adlayer was first partially oxidized at 0.8 V, which is controlled by the attack of oxygen species toward the Ru(0001) surface. The remaining CO adlayer oxidation at 0.55 V is related to the combination of CO molecules with oxygen species already located on the surface. in contrast, successive peaks on RuO2(100) at 0.4 V and 0.72 V are observed, which shows that CO molecules can directly react with two different lattice-oxygen on the surface to carbon dioxide.